An AE (acoustic emission) sensor has conventionally been used to measure elastic waves (elastic vibration) of infinitesimal amplitude. Piezoelectric elements are normally used in an AE sensor. With this technology, it is possible to carry out high precision measurement by amplifying amplitude using resonance of the piezoelectric elements. However, since this technology uses resonance, there is a problem that the frequency band that can be measured is narrow.
It has also been proposed to apply laser Doppler optical fiber sensors to AE sensors. The principle of this method will be described briefly. First of all, a light source is connected to one end of the optical fiber. A reflector for reflecting input light and returning it to the fiber is attached to the other end of the optical fiber. If vibrations are applied to the optical fiber, the optical path length within the fiber will vary with expansion and contraction of the fiber. If time variation of the path length is made dL/dt, light reflected at the end of the fiber varies in frequency in direct proportion to dL/dt due to the Doppler effect. As a result, it is possible to measure vibration by measuring frequency variation between the light reflected from the fiber end and the input light. This type of sensor has a wide bandwidth, but suffers from low sensitivity.
The inventors have observed variations in frequency between input light and output light that has passed along an optical fiber, after curving the optical fiber and applying vibration to the curved section. As a result, the conclusion was reached that frequency variation occurs in the curved section in response to infinitesimal vibrations.
The present invention has been conceived based on the above conclusion, and has as its object to provide a vibration measurement device and method for measuring vibration over a wide bandwidth and with high sensitivity, while having a simple structure.